As a compressed combustible gas produced by compressing a predetermined combustible gas, there are known e.g. a compressed natural gas (CNG) produced by compressing a natural gas containing methane as its principal component and a compressed hydrogen gas produced by compressing hydrogen.
In recent years, automobiles mounting an engine using such compressed combustible gas as fuel have been gaining popularity to cope with the environmental issue, since they produce much less amounts of carbon dioxide, nitrogen oxides, sulfur oxides, particulate matters, etc., as compared with automobiles mounting engines using gasoline or the like as fuel.
An automobile running on the compressed combustible gas as fuel needs to receive supply of the compressed combustible gas by running to a gas filling station, just like the automobile running on the gasoline fuel. And, at this gas filling station, a compressible combustible gas containing combustible gas as its principal component is compressed by a compressor to a very high pressure (e.g. about 20.3 MPa (200 atm)) to be processed into compressed combustible gas and this compressed combustible gas is charged to a high-pressure tank mounted on the automobile (see. Patent Document 1).
For instance, coal mine gas mined in a coal mine contains air components (mainly, nitrogen, oxygen, carbon dioxide) in addition to methane as the combustible gas. In this way, naturally occurring combustible gases are often mined as (raw) material gases containing air components in addition to the combustible gas. In order to effectively utilize such material gases as fuels, it is needed to remove such air components contained in the material gas and to concentrate the combustible gas contained therein.
Especially, if this material gas is to be processed into a compressible combustible gas which is compressed by a compressor into the compressed combustible gas usable as the automobile fuel, for instance, in order to prevent occurrence of e.g. ignition due to the compression, it is needed to remove oxygen contained in that material gas to an extremely low concentration (e.g. lower than 4%).
That is, safety regulation is enforced which provides oxygen concentration upper limits for ensuring safety in the compression in the production process of compressed combustible gas to 4% for such combustible gases as methane, excluding acetylene, ethylene and hydrogen, and to 2% for acetylene, ethylene and hydrogen.
As a concentrating device for concentrating methane by removing the air components from the coal mine gas, there is known an adsorption type concentrating device utilizing an adsorbent formed of natural zeolite that preferentially adsorbs the air components than methane (see. e.g. Patent Document 2).
Further, the adsorption type concentrating apparatus disclosed in Patent Document 2 includes an adsorption tower charged therein with the adsorbent preferentially adsorbing the air components other than the combustible gas and the device is configured to effect in alternate repetition, an adsorbing operation for pressure-feeding material gas such as coal mine gas at a relatively high pressure into the adsorption tower and causing an adsorbent material to adsorb preferentially the air component contained in the coal mine gas and a desorbing operation for discharging concentrated gas containing a large amount of combustible gas which was not adsorbed by the adsorbent or which has been preferentially desorbed from the adsorbent, by depressurizing the inside of the adsorption tower to the atmospheric pressure.
Patent Document 1: Japanese Patent Application “Kokai” No. 9-264196
Patent Document 2: Japanese Patent Application “Kokai” No. 58-198591